Light Regulation of Diurnal Variation of Sucrose-Phosphate Synthase in Potato

Light regulation of diurnal variation of sucrose-phosphate synthase (SPS) activity was studied in potato leaves during pre-tuber initiation stage. SPS activity was greatest at 14:00 and coincident with highest irradiance 2 000 μmol(PAR) m-2 s-1], and lowest at 18:00 during lowest irradiance (200 μmol m-2 s-1). In contrast, fructose-1,6-bisphosphatase (FBPase) activity was greatest at 12:00 (irradiance 1 700 μmol m-2 s-1). Inhibition of SPS activity by phosphorylation was least at 14:00 and highest at 18:00. Considerable inhibition was also observed at 10:00. SPS showed hyperbolic saturation kinetics in response to varying UDP-glucose concentrations. Diurnal variation of SPS activity was due to a change of affinity for its substrate (Km), but not due to change of Vmax. Phosphate (Pi) decreased the activation state of SPS. Inhibition by Pi was most severe under limiting UDP-glucose concentration. Sensitivity towards Pi inhibition at limiting substrate concentration was highest at 18:00 and lowest at 14:00. Change of tissue phosphate concentration followed the opposite pattern to that of diurnal variation of SPS activity and irradiance, being highest at 18:00 and lowest at 14:00. A dual mode of regulation is considered to exist for diurnal variation of potato leaf SPS: covalent modification and effector (Pi) regulation. Thus the change of tissue phosphate concentration may play a pivotal role in regulating potato leaf SPS. This revised version was published online in July 2006 with corrections to the Cover Date.